(a) Field of the Invention
The present invention relates to a differential interference microscope so adapted as to observe a light ray from an object through a compensator and an analyzer while irradiating the object with a light ray emitted from a light source through a polarizer and a beam splitter, and to polarizing optical elements such as a beam splitter and compensator to be used in the differential interference microscope.
(b) Description of the Prior Art
The conventional transmission type differential interference microscope consists, as illustrated in FIG. 1, of a light source 1, a polarizer 2 for linearly polarizing the light ray emitted from the light source 1, a first Wollaston prism 3 (beam splitter) for delicately splitting said linearly polarized light ray into the ordinary light ray and the extraordinary light ray, a condenser lens 4 for making said ordinary light ray and the extraordinary light ray parallel to each other for irradiation of an object M, an objective lens 5 for converging the ordinary light ray and the extraordinary light ray having been transmitted through the object M, a second Wollaston prism 6 (compensator) for composing the ordinary light ray and the extraordinary light ray, an analyzer 7 for obtaining an image consisting of the composed light rays, i.e., a differential interference image by allowing interference between the two polarized light components which have passed through portions of the object M slightly deviated from each other and have been composed by the second Wollaston prism 6, and an eyepiece lens 8 for observing this image.
However, the conventional differential interference microscope described above has a defect that, as shown in FIG. 2 the visual field thereof is not darkened completely but a black striped cross image is formed within the visual field even when no object is placed on the sample stage, thereby degrading uniformity of the visual field. This black striped image is formed for the reason described below. The Wollaston prisms 3 and 6 are ordinarily made of a birefringent uniaxial crystal such as quartz. In case of a plane parallel plate made of quartz cut out in the direction parallel to the optic axis thereof as illustrated in FIG. 3, the ordinary light ray and the extraordinary light ray travel at different velocities in quartz, thereby differentiating optical path length. When the difference in optical path length is represented by R, it is approximated as follows: ##EQU1## wherein the reference symbol i represents angle formed between the light ray incident on the plane parallel plate and the optical axis, the reference symbol .theta. designates azimuth between the incident light ray and the optic axis, the reference symbols n.sub.e and n.sub.o denote refractive indices of the substance composing the plane parallel plate for the ordinary light ray and the extraordinary light ray respectively, the reference symbol n represents mean value of the refractive indices for both the light rays (n=(n.sub.e +n.sub.o)/2), and the reference symbol d designates thickness of the plane parallel plate as measured on the optical axis. In order to cancel the difference in optical path length R, each of the Wollaston prisms 3 and 6 is designed as a combination of two wedge-shaped prisms arranged in such a manner that the optic axes thereof are perpendicular to each other. When the two prisms are combined in this manner, the ordinary light ray and the extraordinary light ray for one wedge-shaped prism are the extraordinary light and the extraordinary light for the other wedge-shaped prism, respectively, whereby the difference in optical path length produced in one wedge-shaped prism is cancelled by the other wedge-shaped prism. However, this design is effective for cancelling only the term of (n.sub.e -n.sub.o)d in the formula (1) mentioned above. Due to cos 2(.theta.+90.degree.)=cos (2.theta.+180.degree.)=-cos 2.theta., the term including i.sup.2 cos 2.theta. is arithmetically composed and remains without being cancelled, thereby producing ununiformity of the difference in optical path length, i.e., the striped cross image and ununiformalizing the visual field.
Solution of this problem was attempted by U.S. Pat. No. 3,904,267. However, the differential interference microscope disclosed by this patent comprises an additional compensator made of a birefringent substance for cancelling the term including i.sup.2 cos 2.theta., i.e., the ununiformity of the difference in optical path length and requires an increased number of optical elements, thereby posing another problem that the microscope occupies a wider space and requires higher manufacturing cost.